As office automation has advanced and office equipment become more sophisticated, the use of color imaging has grown accordingly. In particular, electrophotographic image forming apparatuses such as copiers and laser printers have become ubiquitous. One type of such electrophotography equipment known generally as a cleaner-less image forming apparatus has been proposed, which eliminates the usual cleaning means that typically consists of a cleaning blade for cleaning toner from an image carrier of the image forming apparatus. The advantage of such an arrangement is that it does away with the need for such maintenance work as disposing of any residual transfer toner. With image forming apparatuses of this type, any residual toner remaining in the unexposed portions of the image carrier is returned to the toner container inside the developer unit by electrostatic action, that is, by the difference between the electric potential of the surface of the image carrier and the development bias imparted to the developer roller. In addition, elimination of the blade or other such cleaning means makes it possible to make the image forming apparatus itself more compact and easier to produce.
However, picture quality deteriorates whenever this sort of cleaner-less process is applied to multiple full-color electrophotography systems, for the following reason:
In the case of a full-color printer, the color image is printed onto the recording paper atop the transfer belt and the transfer roller that together make up a single transfer member by contacting the recording paper with successive photoreceptors of different colors, so that the final printed product represents successive overlapping (that is, overlaid) color toner images. But in so doing, toner transferred from the upstream photoreceptors adheres first to the recording paper and from the recording paper to the downstream photoreceptors. For example, in an arrangement in which image formation is performed in the order of yellow (Y), magenta (M), cyan (C) and black (BK), yellow toner transferred upstream can adhere to the downstream magenta toner photoreceptor, the cyan toner photoreceptor and the black toner photoreceptor.
With the cleaner-less process, the off-color toner, that is unusual toner, that adheres to the downstream photoreceptor is then returned to the toner container, thus changing the color of the toner contained in the toner container due to this inclusion of off-color toner, which then changes the coloration of the toner transferred to the recording paper and thereby degrading the resulting picture quality.
Also, apart from the off-color toner, the toner contained in the toner container inside the redevelopment unit contains a large amount of degraded toner (that is, defective toner), which is toner that has not received a sufficient electrical charge during electrostatic charging. For example, as image formation continues and the amount of toner inside the toner container decreases and becomes small, the amount of degraded toner remaining in the toner container becomes large. Since this degraded toner has only a weak charge, it is hard to hold at the developer roller, causing the amount of coating on the developer roller to become uneven. This unevenness in the coat roller can in turn cause the degraded toner to harden into lumps which, when developed, form blotches on the image.